The present invention relates to a display apparatus and a display reading apparatus capable of displaying high-quality images and reading such images at lower cost.
Recently, organic EL (Electro Luminescent) devices are attracting attention as components employed to constitute a display apparatus. In general, a TFT (Thin Film Transistor) is used for driving an organic EL device, but a problem arises of deterioration in the image quality when controlling the driving of the TFT by “voltage”, due to large variations in the characteristics (particularly the threshold voltage Vth) of the TFT.
In order to reduce such variations, a current programming method (as disclosed in, e.g., cited Patent Document 1) is proposed which controls driving of the TFT by “current” that causes less harmful influence derived from the transmission path.
Referring to FIGS. 1 through 3, an explanation will be given below on a method of driving an organic EL device according to a current program. FIG. 1 is a schematic diagram showing a circuit 1 to control light emission of an organic EL device by a current program. In the circuit 1, a capacitor 13 and a source (S) terminal of a driving transistor (TFT) 12 are connected to a source line 18. The capacitor 13 is connected to a gate (G) terminal of the driving transistor 12 while being connected to a model current source 14 via a switch A. A drain (D) terminal of the driving transistor 12 is connected to the model current source 14 via a switch B, and an organic EL device 11 is connected to the drain (D) terminal of the driving transistor 12 via a switch C.
According to the current program in the circuit 1, the switches A to C are set as shown in FIG. 2, so that a predetermined model current I1 obtained from the model current source 14 is caused to flow in the circuit 1. Then the current I1 flows in the driving transistor 12 to thereby generate, at the gate thereof, a gate voltage corresponding to the current I1. And the gate voltage thus generated is charged in the capacitor 13. The current program is executed in this manner.
Meanwhile, in the case of activating the organic EL device 11 for emission of light according to the current program, the switches A to C are so set as shown in FIG. 3. In this case, the model current source 14 is disconnected from the circuit 1, and the driving transistor 12 causes a flow of the current I1, which corresponds to the voltage charged in the capacitor 13, into the organic EL device 11. As a result, the organic EL device 11 is activated to emit light therefrom.
The switches A to C shown in FIG. 1 may be replaced with switching transistors as well. FIG. 4 shows another example of forming the circuit 1 by replacement of the switches A to C in FIG. 1 with switching transistors 25 to 27.
Thus, it is possible to generate a predetermined current and to activate the organic EL device to emit light at an intensity corresponding to the current.
Patent Document 1:
Japanese Patent Laid-open No. 2002-189445
However, for constituting a display apparatus of a high image quality, it is necessary to cause a flow of a minute current in the organic EL device. According to the technology disclosed in the above Patent Document 1, a problem exists where a minute current fails to be programmed accurately and rapidly. Further, another problem exists where, in enabling the organic EL device to read the image by receiving the light, another driving circuit needs to be provided separately from the aforementioned driving circuit used to display the image, hence increasing the production cost.